Process for reducing the density of silver images and compositions therefor



Patented Apr. 2, 1940 UNITED STATES PROCESS FOR REDUCING THE DENSITY FSILVER IMAGES AND COMPOSITIONS THEREFOR David M. McQueen and Max T.Goebel, Wil mington, Del., assignors to Du Pont Film ManufacturingCorp., New York, N. Y., a corporation of Delaware No Drawing.

Application October 3, 1938;

Serial No. 283,117

17 Claims.

This invention relates to photography; more particularly it relates tothe processing of light sensitive photographic elements; still moreparticularly it relates to photographic processes for -'5 reducingand/or removing silver images. The invention also relates to processingbaths for use in the aforedescribed processes.

This invention has for an object new and efficient methods of processingphotographic ele- 10 ments. A further object is to provide a rapid andefficient reversal process for treating photographic film. A stillfurther object is to provide an improved process for reducingphotographic silver images. A still further object is to modifychemically silver images so that they may be rapidly and completelyremoved from photographic elements. Still other objects will appearhereinafter.

The above and other objects are accomplished 20 by the followinginvention which comprises the use of a sulfamic acid in a photographicproc- The silver sulfate, being appreciably water-sol- I uble, can beeliminated by washing and, depending upon the time of contact with thebath, the silver image is reduced or completely removed. Procedures ofthis type have come to be very important in the processing of certaintypes of films, e. g. sub-standard size motion picture films and certaintypes of color films. In preparing a direct positive from a negativeimage by the reversal method, the negative image may be treated with abath of potassium permanganate or potassium dichromate and sulfuric acidto remove 50 the silver image and the residual silver halide and thenre-exposed and developed.

It has now been found that the sulfamic acids, particularly sulfamicacid, can be advantageously used to replace sulfuric acid in a reversal"bleaching bath and that superior and unexpected results are obtained bysuch substitution. Bleaching as herein used refers to a process by whichsilver is converted to a silver salt which is usually an appreciablywater soluble salt which is thereupon removed from the photographicfilm. It

More particularly the has been found that the silver sulfamates,particularly silver sulfamate, are exceedingly more soluble in the usualphotographic and washing baths. This increased solubility is .a distinctadvantage since it means that silver images may be more completelyremoved. The action of bleach baths containing a sulfamic acid,particularly sulfamic acid, furthermore, have been found to be morerapid than the sulfuric acid which is usually employed.

The invention will be further illustrated but is not to be limited bythe following examples in which the parts stated are parts by weight:

Example I A negative motion picture film was exposed and given thefollowing treatments:

(1) Development for 8-9 minutes in a developer of the followingcomposition:

Parts Water," 1000 Metol 1.5 Hydroquinone 1.5 Anhydrous sodium sulfite8.0 Potassium thiocyanate 3.0 Potassium bromide 3.0 Potassium carbonate20.0

(2) Five minutes washing in running water (3) Bleaching for 3-4 minutesin a bath of the following composition: 4

Parts Water 1000 Potassium dichromate 4.5 Sulfamic acid 15.8

(4) Washing in running water for 5 minutes; (5) Clearing for 5 minutesin a bath of the following composition:

Parts Water 1000 Sodium bisulflh: 20

Parts Water 000 Mgtnl v 1 Hydroquinone 6 Anhydrous sodium sulfite 25Potassium br l Potassium carbonate 40 omitted.

Example II A negative motion picture film was exposed and then given thefollowing treatments:

(1) Development for approximately 5 minutes at 18 C. in a bath of thefollowing composition:

Parts Water 4500 Me 13 Anhydrous sodium sulfite 100 Potassium bromide 5.5 Ammonium hydroxide (sp. gr. 0.90) 2'7 Hydroquinone 4 (2) Washing for5-10 minutes in running water;

(3) Bleaching for 3 minutes at 18 C. in a bath of the followingcomposition:

Parts Water 1325 Sulfamic acid 17.5 Potassium dichromate 3 (4) Washingfor 5 minutes in running water; (5) Clearing for 6 minutes in a bath ofthe following composition:

Parts Water 1000 Anhydrous sodium sulfite 100 (6) Washing for 8 minutesin running water; ('7) Exposure to diffuse daylight for one minute;

(8) Development for 5 minutes at 18 C. in a bath of the followingcomposition:

Parts Water 1000 Metol 2 Hydroquinone 4 Anhydrous sodium sulfite 25Sodium carbonate 18. 5

Potassium bromide 2 (9) Washing for ten minutes in running water. Bythis procedure a very clean positive of excellent gradation wasobtained.

Example III A color film of the ruled screen type was exposed andprocessed as follows in total darkness:

(1) Development for 4 minutes in a bath of the following compositionParts Metol 6.5 Hydroq-uinone 1. 25 Sodium sulfite 100 Sodium carbonatePotassium bromide 2. '75 Potassium thiocyanate 9 Water 790 (2) Treatmentfor 2 minutes in a 1% solution of acetic acid;

(3) Washing for 5-10 minutes in running water;

(6) Clearing in a 2.5% solution of potassium metabisulfite (7) Washingin running water for 2 minutes;

(8) Exposure for 3-4 minutes to a watt lamp at a distance of 3 feet;

(9) Development developer;

I color, as described for 6 minutes in the original (10) Washing inrunning water for 4 minutes; (11) Treatment for 2 minutes with a 40%solution of sodium thiosulfate;

(12) Washing in running water for 15 minutes- By this procedure anexcellent color transparency of good detail and color contrast wasobtained.

Example IV A negative was made by exposing a multilayer film, that is, afilm coated with emulsions of different spectral sensitivities as iscommon in certain processes of color photography. In this case the filmwas composed of a panchromatic emulsion deficient in green sensitivitycoated over cellulose acetate base, a clear gelatin layer, anorthochromatic emulsion, a layer of gelatin dyed yellow and, on top, alayer of emulsion sensitive only to blue and ultraviolet light. This wasdeveloped in a metol-hydroquinone developer and thoroughly washed. Thesilver deposits were then bleached with a bath of the followingcomposition:

, Parts Water 1000 Potassium dichromate"; 6 Sulfamic acid 20 and clearedin a bath of two per cent sodium bisulfite solution.

The silver was found to be removed completely and no stain was left inthe film. This film is suitable for further processing to pictures in inthe art.

In place of sulfamic acid itself which is described in the specificexamples, other sulfamic acids may be substituted. However, it is to bedistinctly understood that sulfamic acid is the prepared reagent and hassuperior photographic properties. As examples of suitable acids, mentionis made of the lower monoand di-alkyl sulfamic acids, e. g.mono-methyl-sulfamic acid, mono-ethyl-sulfamic acid, di-methylsulfamicacid, di-ethylsulfamic acid; methylolsulfamic acid, etc. The easilyhydrolyzable salts, e. g. aluminum sulfamate may also be used.

The sulfamic acids may be incorporated in the bath over a fairly widerange of proportions. For instance, 2 to 30 grams of sulfamic acid perliter of water represents a practical range and 5 to 10 grams per litera preferred range. Similar amounts of the substituted sulfamic acids maybe used.

The amount of oxidizing agent used may likewise vary over a fairly widerange. For instance from 25 to 2 grams per liter of potassium dichromateor potassium permanganate represents a practical range and 10 to 2represents a preferred range. Similar amounts of other oxidizing agent:may be used.

In place of the oxidizing agents set forth ir. the preceding examples,any oxidizing agen1 which will aid in solubilizing the silver so that iiwill form a sulfamic acidsalt and which does n01 destroy the sulfamicacid used may be substitutec for those set forth in the examples. Theoxidizing agents preferably should not have an adversl effect on thegelatin binder. As examples of othel utilizable oxidizing'agents,mention is made 0: ferric ammonium sulfate, ceric ammonium sul fate,ammonium persulfate, sodium dichromate sodium permanganate,ammoniumdichromate and ammonium permanganate. Mixtures of 'thabove-described agents including those of the ex amples may be used. Thealkali metal 'dichro mates and permanganates give the best results andrepresent the preferred embodiment hereof.

The processing baths of the present invention comprising in itspreferred form sulfamic acid and potassium dichromate or potassiumpermanganate are not limited in their use to reversal bleachingprocesses but may be used in various types of processes wherein aphotographic reducer is required. Thus, if the concentration of sulfamicacid and oxidizing agent is materially reduced, the resulting bath maybe used to lessen the density of films containing silver images. Theperiod of processing is generally less than that required for a reversalprocedure.

The baths may be used with practically any type of photographic elementcontaining a silver image. For instance, they may be used in conjunctionwith motion picture film, either for color or black and whitephotography, still camera films or plates, permanent record filmsuitable for the use of recording business, library and historicaldocuments, and non-motion picture film which is sometimes processed byreversal in order to obtain fine grain. They may also be used inconjunction with direct paper print cameras and copying machines.

The use of the bleaching baths of the present invention, furthermore,-isnot to be limited by the other steps set forth in the examples. Forinstance, any normal type of developer may be used, and the exposureafter bleaching may be controlled or carried to complete blackening.

The bleaching baths of the present invention may contain besides theabove-mentioned constituents, other agents, such as materials added todecrease the penetration of the bath, such as alcohol, acetone andglucose; softening agents for the photographic film and emulsion, suchas glycerol; hardening agents, e. g. inorganic salts with no specificeffect on silver images or silver halides.

The removal of silver from photographic films has been found desirablein a number of methods of processing. As hereinbefore mentioned, thecustomary procedure has been to employ a bath of oxidizing agent whichhas no deleterious action on the gelatin of the emulsion, and sulfuricacid. Attempts to supplant sulfuric acid have not been successful, andits use has become more or less standard in the industry. Thus,replacement by nitric acid, the silver salts of which are more solublein water than silver sulfate has not proven to be successful. Aceticacid has been proposed but is not satisfactory for a number of economicreasons, including the fact that its slow speed renders it practicallyuseless.

It therefore could not be predicted from the acids just described thatthe sulfamic acids, particularly sulfamic acid, could be advantageouslyused in conjunction with certain oxidizing agents to remove silver.Silver images are removed rapidly and completely by the use of suchacids. This is an economic advantage in that it saves valuableprocessing time. A further advantag'resides in the fact that lessprocessing equipment such as washing equipment in continuous processesis required. Furthermore, the use of sulfamic acid leads to cleanerfilms and results in finer picture detail.

Since sulfamic acid is a nonhygroscopic, crystalline solid its usepresents marked advantages in convenience of handling as compared withsulfuric and other acids. It can be readily weighedout without specialprecautions and furthermore does not present the corrosive hazardinvolved of the silver after the first development is very 7 oftendifficult and incomplete. The use of these acids as bleaching agentsresults in increased color contrast of the finished pictures and in animprovement in definition which is of great importance in color work.The outstanding rapidity 3 v of action of baths comprising sulfamic acidand an oxidizing agent is also peculiarly suited for use with filmswhich demand rapid processing, as for example, in the processing ofrecords of business documents, checks, etc.

As many apparently widely difierent embodiments of this invention may bemade without departing from the spirit and scope thereof, it is to beunderstood that we do not limit ourselves to the specific embodimentsherein except as defined by the appended claims.

We claim:

I. An aqueous photographic bleach bath comprising an oxidizing agent anda compound taken from the group consisting of sulfamic acids and theireasily hydrolyzable salts.

2. An aqueous photographic processing bath containing a sulfamic acidand an oxidizing agent.

3. A photographic processing bath comprising an aqueous solutioncontaining sulfamic acid and an oxidizing agent.

4. A photographic bleach bath comprising an aqueous solution containingfrom 0.2 to 3% of sulfamic acid and 2.5 to 0.2% of an oxidizing agent.

5. A photographic bleach bath comprising an aqueous solution containingsulfamic acid and an alkali metal dichromate.

6. A photographic bleach bath comprising an aqueous solutioncontainingsulfamic acid and ammonium persulfate.

'7. A photographic bleach bath comprising an aqueous solution containingsulfamic acid and an alkali metal permanganate.

8. A process for reducing the opacity ofa silver deposit which comprisestreating the same in an aqueous processing bath containing a sulfamicacid and an oxidizing agent.

9. A process which comprises reducing the density of the silver image ofa photographic element by treating theelatter in an aqueous bathcontaining an oxidizing agent and a compound taken from the groupconsisting of sulfamic acids and their easily hydrolyzable salts.

10. A process which comprises reducing the density of the silver imageof a photographic element by treating the latter in an aqueous bathcontaining a sulfamic acid: and an oxidizing agent.

11. A process which comprises reducing the density of the silver imageof a photographic element by treating the latter in an aqueous bathcontaining sulfamic acid and an oxidizing agent.

12. In a process of producing positives by a reversal method the stepwhich comprises treating a photographic element containing a silverimagewith an aqueous bath containing a sul iamic acid and an oxidizingagent.

13. In a process of producing positives by a velopment of the reversedimage.

16. A photographic reversal process comprising exposure to an image,development in an ordinary photographic developer, bleaching in aprocessing bath containing suliamic acid and an oxidizing agent andprocessing the reversed image to pictures in color.

17. A photographic reversal process comprising exposing a. multilayerfilm coated with emulsions of different sensitivity, developing thelatent images in an ordinary photographic developer, bleaching thedeveloped film in a processing bath containing sulfamic acid and anoxidizing agent and processing the reversed images to pictures incolor.-

' DAVID M. McQUEEN.

MAX T. GOEBEL.

